In recent years, in the field of information communication devices such as mobile phones, acoustic wave devices having a comb-shaped IDT electrode formed on a surface of a piezoelectric substrate are used as circuit elements such as resonators, filters, and the like. FIG. 26 shows examples of such acoustic wave devices. In (a) of FIG. 26, a top view of an acoustic wave device 900 is shown. The acoustic wave device 900 is formed by arranging two IDT electrodes 902 and two reflectors 903 on a piezoelectric substrate 901. The IDT electrodes 902 each have a bus bar 911 and a plurality of electrode fingers 912 that extends from the bus bar 911. The respective electrode fingers 912 of each of the IDT electrodes 902 are arranged such that the electrode fingers 912 thereof are arranged interleaved with the respective electrode fingers 912 of the other IDT electrode 902. The reflectors 903 are arranged so as to interpose the IDT electrodes 902 therebetween. In (b) of FIG. 26, a top view of another acoustic wave device 950 is shown. The acoustic wave device 950 is different from the acoustic wave device 900 in that dummy electrode fingers 913 that extend, from the bus bar 911, alternately with the electrode fingers 912 are further provided, and in that the electrode fingers 912 are arranged so as to face the dummy electrode fingers 913 of the other IDT electrode 902. In each of the acoustic wave devices 900 and 950, a strip-shaped intersection area within the region in which the electrode fingers 912 are arranged interleaved with each other, excluding end portions of the respective electrode fingers 912, is used as a main propagation path of an acoustic wave.
Patent Literature 1 discloses an acoustic wave device 1000 which is different from the acoustic wave device 900 in that a waveguide is formed by providing a coating film 1004 that serves as a piezoelectric body or an insulator in the intersection area. FIG. 27 is a sectional view of the acoustic wave device 1000 aligned along a single electrode finger 912. In the acoustic wave device 1000, propagation of an acoustic wave is concentrated to the coating film 1004, thereby suppressing a high-order transverse mode wave which is an unnecessary wave.
Patent Literature 2 discloses: an acoustic wave device 1100 which is different from the acoustic wave device 900 in that, in each of two strip-shaped edge areas adjacent to the intersection area, including end portions of the respective electrode fingers 912, the electrode fingers 912 of the IDT electrode each have an increased width and an increased film thickness; and an acoustic wave device 1150 which is different from the acoustic wave device 900 in that a dielectric film 1154 is provided on the IDT electrode only in the edge areas. In (a) of FIG. 28, a top view of the acoustic wave device 1100 is shown. In (b) of FIG. 28, a sectional view of the acoustic wave device 1150 aligned along a single electrode finger 912 is shown. It is generally known that, in an acoustic wave device, acoustic velocities in edge areas and bar areas are decreased so as to be less than an acoustic velocity in an intersection area. Accordingly, in the intersection area that serves as a main propagation path, a fundamental transverse mode wave can be confined and a high-order transverse mode wave which is an unnecessary wave can be suppressed. However, in Patent Literature 2, acoustic velocities in the edge areas are controlled by increasing end portions of the electrode fingers of the IDT electrode in size and mass.
Patent Literature 3 discloses an acoustic wave device 1200 which is different from the acoustic wave device 900 in that a dielectric film 1204 covers the piezoelectric substrate 901, the IDT electrodes 902, and the reflectors 903 such that the surface thereof is flattened. FIG. 29 is a sectional view of the acoustic wave device 1200 aligned along a single electrode finger 912. In the acoustic wave device 1200, the dielectric film 1204 covers the piezoelectric substrate 901, thereby reducing frequency characteristic change of the acoustic wave device 1200 depending on temperature.